Make sure we have guards on header validity for all tables

[ccascone]

Like we did for issue #97 in PR #99, we should verify that we have guards on header validity in all other tables of the pipeline.

I wish the compiler would be capable of emitting warnings for such conditions.

[ccascone]

We added some guards in #194 for the obvious cases.

We should make sure to add some protection for tables using ternary match like the ACL one by either:

• Adding hdr.isValid() to the match key and making sure the control plane sets that appropriately. E.g, table { key {ipv4.isValid(): ternary; hdr.ipv4.src: ternary; ... } }; or
• Matching on metadata that is properly initialized before applying the table, instead of matching directly on headers. E.g., in the ACL table use the following to match on the IPV4 source address lkp_ipv4_src = hdr.ipv4.isValid() ? hdr.ipv4.src : 0.